Self-propelled harvesting machines, as combines, forage harvesters or cotton pickers, are usually propelled by a hydrostatic motor which is in hydraulic connection with a pump driven by a combustion engine. Such a harvesting machine is described in EP 0 819 562 A. There, the output speed of the hydrostatic motor is determined by a operator by means of an operating lever or drive lever mechanically controlling the position of a swash plate of the pump. The hydrostatic motor is driving the wheels of the harvesting machine via a gearbox with a transmission ratio selected by the operator. The speed range of the harvesting machine that can be obtained by moving the operating lever along its available path depends on the selected transmission ratio. Hence, in the first transmission ratio, a smaller speed range is achieved as in the third transmission ratio. Thus, independent of the selected transmission ratio, at least almost the full moving range of the operating lever is available for changing the propelling speed. A disadvantage however is that the operator has to perform the transmission ratio selection manually and that the machine has to stop before another transmission ratio can be selected.
Another drive system for a self-propelled harvesting machine is described in EP 1 431 619 A. There, the hydrostatic motor drives the wheels via a gearbox enabling a transmission ratio change even while the machine is driving. The position of the swash plate of the pump and of a swash plate of the hydrostatic motor are determined by an electronic control unit receiving a desired speed input signal from a potentiometer connected to the operating lever. In a work mode, only the first transmission ratio is selected and the moving range of the operating lever corresponds to the entire speed range for the work mode. In a transport mode, a first and a second transmission ratio are automatically selected and the moving range of the operating lever corresponds to the entire speed range for the transport mode. The control unit obtains input regarding the operating mode from an operator or derives the operating mode from the state of work elements of the machine. Here, only two speed ranges of the operating lever are available, such that the propelling speed of the machine cannot always be controlled as sensitively as desired, for example when the machine is in the transport mode, but driving slowly downhill or uphill or on narrow lanes.